Fiber pulp, such as waste fiber, contains not only fiber-like raw materials, but also filling agents, such as ash and also fines originating from the fiber-like material. To be able to reuse such a fiber pulp in paper-making, it needs to be washed in a de-inking plant. There, a certain part of the fine fraction of the fiber pulp is removed by means of water, and the aim is to leave only useful fibers in the pulp. Thus, ash and other particle-like materials, for instance, are washed away by means of filtrate waters. Further, when washing fiber pulp, water is removed from it with the aim of affecting the consistency of the washed pulp. The washed accept is led on in the process to be used as raw material for newsprint and soft tissue, for instance.
There are several different apparatuses on the market for the above-mentioned pulp washing. The known apparatuses remove fines relatively well. The problem is, however, that fines loss cannot be adjusted during operation in the present apparatuses, and such an adjustment always requires a shutdown. When process conditions change, the apparatus is stopped and washing is changed by changing the wires, for instance. A further problem may be that when attempting maximum ash removal, fines loss becomes too high, which is uneconomical with respect to efficient use of the pulp. Changes in the process impair the running of the washers, producing a varying washing result which then causes problems in the phases after the washers.
The object of this invention is to provide a method and an arrangement which prevent the problems occurring in prior art solutions.